In recent years, along with the reduction in size and weight and the performance enhancement of mobile phones, notebook computers, electric cars and the like, lithium-ion batteries having a light weight and a large charge capacity are broadly utilized as secondary batteries to be used for these apparatuses. Then in the applications to electric cars, electric power tools and the like, the insufficiency in the large current load characteristics is a big problem. As countermeasures thereto, there are proposed, particularly for negative electrodes, the resistance reduction in the electrodes and the adoption of materials excellent in the rate characteristics, such as hard carbon and soft carbon, to active substances.
For example, Patent Document 1 (JP2010-129169A) discloses a negative electrode material which is obtained by mixing a carbon nanotube and a thermoplastic resin and heating them in an inert gas to thereby coat the carbon nanotube with carbon by the pyrolysis, for the purpose of suppressing an increase in the initial irreversible capacity when a carbon nanotube having a possibility of producing a larger reversible capacity than graphite is used as an anode material.
Further Patent Document 2 (JP2010-123437A) discloses that by mixing a graphite material of a negative electrode with a carbon nanohorn aggregate as an auxiliary conductive agent, there can be obtained a long-life lithium-ion battery which is low in the reaction resistance and low in the volume expansion coefficient, and causes no rapid capacity deterioration.
Patent Document 3 (JP2008-66053A) discloses a method for fabricating a nanotube integrated with hard carbon (HC) or soft carbon (SC), the method involving mixing a precursor of carbon particles to become a core with a metal-containing compound and heat-treating the mixture, and also discloses lithium-ion battery characteristics.
Further Patent Document 4 (JP2006-117451A) discloses a method of obtaining a composite carbonized material. This method involves first subjecting phenols and aldehydes to an addition condensation reaction in the presence of a reaction catalyst while carbon nanofibers and a dispersant are being mixed therewith. Thereby, a phenol resin globularly aggregates while being incorporating the carbon nanofibers to thereby form a carbon nanofiber-phenol resin composite material composed of globular particles of the phenol resin containing the carbon nanofibers homogeneously dispersed therein. It is disclosed that the composite material is further heat-treated to thereby obtain a carbon nanofiber-phenol resin composite carbonized material in which the phenol resin has been carbonized. In this composite carbonized material, the carbon nanofibers are homogeneously dispersed in the carbonized substance of the phenol resin, and it is contended that the effect of improving the electroconductivity can be highly attained by compositing the carbon nanofibers.